1. Field of the Invention
The present invention relates to image data processing devices, which are arranged in digital copying machines, facsimiles and others for compressing image data supplied serially in the bit map format, and also relates to digital copying machines equipped with image memories for storing image data obtained by reading originals with image readers.
2. Description of the Background Art
Digital copying machines can perform copying in the memory mode. In this mode, image information produced by reading an original with an image reader is temporarily stored in an image memory, and a copy image is produced based on the data in the image memory. In the multi-copy processing in the memory mode, an operation of reading an original is required only one time regardless of the number of copies to be produced. Therefore, a copy time, i.e., a time required for copying can be remarkably reduced in the memory mode, if reading of the original requires a longer time than printing. Electronic sort is an application of this memory mode copying. According to the electronic sort, image information is read from a plurality of originals for storing the same in the image memory, and a plurality of sets of copied documents are individually produced by repeating an operation of successively reading out image information of the originals from the memory.
In general, the image memory has stored image information in a compressed format, which will be referred to as "compressed data" hereinafter. The purpose of compression is to increase the number of originals which can be stored. In the conventional copying machine, the image data representing pixels, which are obtained by scanning and reading the original with the image reader, is stored in the image memory in order of arrangement of the pixels, and a compressing operation starts after the image data for one page, i.e., image data corresponding to one sheet of original, is completely stored. Thus, the compressing operation is performed page by page. Scanning for the next original starts after the compression of image data for the last page is completed.
In the copying machine, in which original image data read with the image reader as described above is temporarily stored in the image memory, a copying time is longer by a time required for access to the image memory than that in a so-called analog copying machine, in which image information is directly transferred from an image reader to a printing means, when only one copy is to be produced from one original, i.e., in general copying processing. Thus, the copying speed decreases. Particularly in the structure where image data is compressed for storing the same in the image memory, a time for compression and expansion is additionally required, which further increases a copying time.
In some digital copying machines in which a read original image is temporarily stored in the image memory, a size of an original is first detected to determine a size of an input image memory area in the image memory, and the image data is read into the input image memory area. Also, such digital copying machines have been developed that image data is read into the memory without detecting the size of original, and the size of original is detected from the image data on the memory.
However, in this digital copying machine, in which image data is read into the image memory and the size of original is detected from the image data on the memory, the size of original is not initially determined, so that the memory area of the image memory for reading image data must be set to correspond to the maximum size of original which is readable by the image reader.
As a result, even if the actual size of the original is small and therefore the required input image memory area of the image memory is small, the maximum memory area must be initially prepared for reading the image, resulting in a low efficiency of memory use.
In the structure where an image read onto the input image memory area is held in a compressed format by an image holding memory area, the image stored in the input image memory area which has been set correspondingly to the maximum original size must be compressed together with an unnecessary image area which does not correspond to the original, so that the compression time and therefore expansion time after compression unnecessarily increase.